1. Field of the Invention
The present invention relates to a differential input circuit using a folded cascode amplifier. The differential input circuit according to the present invention is used for a USB (Universal Serial Bus) interface, for example.
2. Description of Related Art
As the miniaturization of MOS transistors advances, the withstand voltage of gate oxide film is dropping. Therefore a power supply with lower voltage is used for an integrated circuit as the degree of integration becomes higher. Generally about 3.3 volts of power supply voltage is used for integrated circuits fabricated in about a 0.35 μm micro process. For integrated circuit fabricated in about a 0.18 μm micro process, about 1.8 volts of power supply voltage is used. In the case of 3.3 volt power supply voltage, the signal level of this integrated circuit vibrates between 0 volts and 3.3 volts. Similarly in the case of a 1.8 volt power supply voltage, the signal level of this integrated circuit vibrates between 0 volts and 1.8 volts. Therefore for interconnecting integrated circuits which have a different degree of integration, an interface circuit for converting the signal level is required.
Normally an interface circuit for converting the maximum potential of a signal from high potential into low potential uses a voltage which matches with the high potential as the power supply voltage. For example, an interface circuit which converts the maximum signal potential from 3.3 volts to 1.8 volts must use a 3.3 volt power supply. Therefore in such an interface circuit, a transistor of which the withstand voltage of the gate oxide film is 3.3 volts must be used.
Conventionally, a circuit using a folded cascode amplification circuit has been known as a differential amplification circuit. As a folded cascode amplification circuit, a circuit disclosed in FIG. 1 of U.S. Pat. No. 4,797,631, for example, is known. According to this folded cascode amplification circuit, a differential amplification circuit with which the distortion of waveforms is small and high frequency operation is implemented can be provided.
As described above, according to prior art, low withstand voltage transistors must be used to increase the degree of integration of an integrated circuit chip, and in order to connect this integrated circuit chip to a chip for which a high voltage power supply is used, a differential input circuit must be constructed with high withstand voltage transistors. In other words, conventionally even when an integrated circuit chip with a high degree of integration is fabricated, the differential input circuit alone must be constructed with high withstand voltage transistors. Therefore in an integrated circuit which has this type of differential input circuit, there were two types of film thickness for gate oxide film, which made the manufacturing process complicated. So to simplify the manufacturing process, a differential input circuit which is comprised of only low withstand voltage transistors and can operate with high power supply voltage is required.
In the folded cascode amplifier disclosed in the above patent document, operating a low withstand voltage transistor with high power supply voltage is not considered. If a voltage higher than the recommended voltage range is applied to a gate oxide film, it is possible that the age deterioration of a transistor is accelerated and the reliability of the integrated circuit drops.